The answer is really simple. To protect your own computer from damage, and to prevent your computer from being used to pass on infections to other machines without your knowledge.
Many people are reluctant to shell out the cash for decent antivirus software. I'll agree that at roughly $50 for a decent antivirus program, it isn't exactly small change. But, let's look at the costs more closely for a moment:
Computer --> $600-$4,000
Monitor --> $250-$1000
Software --> $500-$5000
Internet Connection --> $25+/month
Personal/Business Data --> Priceless
So, on the low end of the scale we are looking at a computer investment similar in price to an older used car, on the higher end, similar to a newer used car.
Maybe I'm wrong, but I'm guessing that you wouldn't buy a used car that had all the locks removed and didn't need a key to start it, and comes with a free "STEAL ME" sign on it too.

Having a computer that does not have up-to-date antivirus protection is absolutely the same. You are simply asking for trouble.
And trouble WILL find you.
How can I be so sure? Simply because that is what viruses, worms, malware etc. are made to do.

Their ONLY job is to find unprotected computers and attack and hijack them without your knowledge. These things are VERY good at what they are built to do, and they NEVER stop. If your computer isn't protected, and you share disks or go online, it will get infected.
The top antivirus software programs cost as little as 2 months internet connection. They all include constant and unlimited automatic updates that keep your system safe and clean.
If you can afford $1500+ on a computer, $300+/year on internet connections, you can definitely afford another $50 once to make it safe (and prevent it from being used to spread viruses and Trojan horses behind your back).
Anything less would be uncivilized!
P. Some people have started to make claims that if you don't use antivirus software you should be held legally responsible if your machine be used as part of an attack (even if you did not know about it).
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